This year has been an extremely active tropical cyclone season for the Atlantic Basin. The next named storm is Laura. In comparison, Hurricane Katrina occurred during the last week in August, which was the record season for the numbers of named storms. Each year I watch for the source of Cahurricanes -- The African Easterly Waves (AEWs) in mid-August. In the tropics, we don't see large sea level pressure changes at the surface so we need to examine the atmosphere at 5000-10000 feet above the ground and depend on models and satellites. The number of actual measurements has fallen to only a handful across much of Africa north of the Equator in recent years. This leads to errors in the forecasts but data assimilation and ensembles of forecasts can help to reduce the error. Still this is a major concern for the people in West Africa.
Typically, between August 15-25 these waves become strong and Saharan air and dust events into a quiet mode. Dusty and warm Saharan air limit AEWs from developing into tropical cyclones because the air is (1) bone dry; (2) super stable; (3) has high shear which does not allow clusters of thunderstorms to last very long. Another ingredient is warm water, which increases to values above 80F by late August. So from a climatology point of view, the peak of the season occurs during the second week in September (Figure 1).
Figure 1: Frequency of Hurricanes (Source NHC)
So what's up for this year. Well first of all the monsoon has been very strong leading to above normal rainfall for much of West Africa for the last 3 months (Figure 2) and also the last month (Figure 3).
Figure 2: Satellite-based percent normal rainfall over the last 90 days (Source NOAA).
Figure 3: Satellite-based percent normal rainfall over the last 30 days (Source NOAA).
The above-normal rainfall is partly due to the AEWs and also fast-moving squall lines which can dump lots of rain in a short period of time. The African Easterly Jet (AEJ) -an zone of fast winds, which is also located at about 10000 feet has shifted further north. Like the North American Jet in the winter season, we find AEWs developing just south of the AEJ. This year the AEJ has been very far north, over the desert like the rainy period of the 1960s. Consequently, I would expect strong AEWs and heavy rain over the Sahel region of West Africa during the next 4-6 weeks.
AEWs over the last week
Three systems over the Atlantic are tied to AEWs that emerged off of Africa in the last week. Two of them have the possibility of forming named storms in the next 5 days. The last system which emerged off the coast of Africa on Saturday is in struggle mode but still might form a TC.
https://www.washingtonpost.com/weather/2020/08/17/atlantic-hurricane-activity/
I typically run the Weather and Research Forecasting model to get a sense of the wave. I examine 850 hPa (4500-500 feet) and 700 hPa (10000 feet) Vorticity which gives us a sense of spin and where AEWs are located. For the strongest AEWs the vorticity centers at 850 and 700 hPa are co-located and closed but normally you don't find a close circulation at 700 hPa. The AEW normally looks like an inverted V pattern at 700 hPa.
Figure 4: 850hPa Vorticity field with red showing counter-clockwise or cyclonic motion for 17 August 2020
Caption: 700hPa Vorticity field with red showing counter-clockwise or cyclonic motion for 17 August 2020
AEWs over the next two weeks
AEWs have been moving across West Africa every 3-4 days on a regular basis bringing plentiful rain to farmers and sometimes flooding. However, the long-term forecast suggests that there will be a tendency for the waves to form near the coast of West Africa in addition to traveling over land. We can see this by tracing north-south winds over time in the model forecast. Alternating series of negative and positive meridional (north-south) winds that move on the diagonal can be identified as AEWs (Figure 4). They show the waves moving from east to west (just follow a blue area and look at the date)
Figure 4: Time series of meridional winds (negative - from north, positive from South) at 700 hPa.
If we roll the 850 vorticity into a movie you can get a sense of the waves moving across West Africa and also developing off the coast of West Africa (Video 1).
Video 1: 2-week (August 15-29 2020) forecast of 850 hPa Vorticity movie
The AEWs also will carry precipitation with them as they move westward which is underestimated in the forecast (Video 2).
Video 2: 2-week (August 15-29 2020) forecast of 850 hPa Vorticity movie with forecasted
radar reflectivity
The African Easterly Jet (AEJ) northward location over the Saharan Desert during the next two weeks, will create new disturbances and assist in generating spin as they move across the Atlantic ocean towards the Caribbean and Western Atlantic (Video 3). Again, we know that when the AEJ is this far north, AEWs and tropical cyclones are typically stronger.
Video 3: 2 week (August 15-29 2020) forecast of 700 hPa winds (AEJ)
Be prepared over the next 8 Weeks
We don't know where potential hurricanes will land, but you must be ready if you live in the Caribbean, along the Gulf of Mexico or Eastern US seaboard and Cabo Verde as this may be a record hurricane season in terms of numbers of storms. Some of these hurricanes may become major hurricanes. There is no reason why you can make sure that you have: (1) non-perishable foods and some stored water; (2) extra cash in the event of a major power outage; (3) have all of your important documents in a safe place that you can grab on a minutes notice; (4) Make sure that prescription drugs are filled for yourself or loved ones and have a contact list; (5) have an evacuation plan and know where shelters are located.
This year COVID-19 is going to make going to shelters particularly challenging. So you have to make a very important decision if the time should come: Which one is more likely to injure or kill me in the short-run (Hurricane or COVID-19)? This is not an easy decision if you have co-morbidities such as diabetes, cardiovascular disease, or kidney disease. But you must make the decision not based on fear but on reality at the moment. If you have to go into a shelter, make sure that you have some extra masks and hand sanitizer to keep you and love ones safe.
Note: If movies do not work, I am sorry. In the process of building a website. Stay tuned..
